Yes, the audio book case will state how many CDs and how long it takes to listen to it. I SWEAR by books on CD! They pass the time wonderfully on long trips, and it gives me a chance to get immersed in a story I probably wouldn't have read myself. Something about hearing a story out loud engages the brain in a primal way--that's how humans experienced stories for most of our existence as a species. I think we are hard-wired to connect with it. I also find that I stay alert listening to them, which is a big thing because I tend to get dozy on long drives.
For future reference, my kids love listening to books on CD too, and our town library has a big collection. We have been able to do 7 or 8 hour drives with them sanely thanks to them, and sometimes we listen to them just when we go around town. You might try it out with them.